1. Field of the Invention
The present invention relates to an exposure apparatus and an exposure method in which a pattern formed on a mask is transferred onto a substrate to expose the substrate therewith, and a method for producing a device based on the use of the exposure apparatus.
2. Description of the Related Art
The photolithography step is provided usually as one of the steps of producing the microdevice including, for example, semiconductor elements, liquid crystal display elements, image pickup devices (for example, CCD (Charge Coupled Device)), and thin film magnetic heads. In the photolithography step, the exposure apparatus is used, in which a reduced image of a pattern formed on a mask or a reticle (hereinafter generally referred to as “mask”, if necessary) is subjected to the projection exposure on a substrate as an exposure objective (semiconductor wafer or glass plate coated with photoresist). In recent years, the reduction projection exposure apparatus (so-called stepper) which is based on the step-and-repeat system or the exposure apparatus which is based on the step-and-scan system is used in many cases.
The stepper is such an exposure apparatus that the substrate is placed on a substrate stage which is movable two-dimensionally, and the substrate is moved in a stepwise manner (subjected to the stepping) by the substrate stage to successively repeat the operation in which each of shot areas on the substrate is subjected to the full field exposure with the reduced image of the pattern of the mask. The exposure apparatus based on the step-and-scan system is such an exposure apparatus that a mask stage on which the mask is placed and a substrate stage on which the substrate is placed are moved mutually synchronously with respect to a projection optical system in a state in which the mask is radiated with a slit-shaped pulse exposure beam, while a part of a pattern formed on the mask is successively transferred onto a shot area of the substrate, and the substrate is subjected to the stepping upon the completion of the transfer of the pattern to one shot area to perform the transfer of the pattern to another shot area.
The exposure apparatus as described above has a plurality of optical sensors (light receivers) for receiving the exposure beam through the projection optical system. Various types of mechanical adjustments and optical adjustments are performed and various types of operation conditions are determined on the basis of the outputs of the optical sensors to optimize the exposure operation to be performed when the substrate is actually exposed. Those provided on the substrate stage include, for example, an uneven illuminance sensor (irradiation irregularity sensor) for measuring the uneven illuminance (light amount distribution) of the exposure beam which has passed through the projection optical system and/or measuring the totalized uneven light amount, and a radiation amount sensor (dose sensor) for measuring the radiation amount (light amount) of the exposure beam which has passed through the projection optical system. The irradiation irregularity sensor as described above is disclosed, for example, in Japanese Patent Application Laid-open No. 08-316133. The dose sensor as described above is disclosed, for example, in International Publication No. 01/008205.
In recent years, it is demanded to realize the higher resolution of the projection optical system in order to respond to the further advance of the higher integration of the device pattern. The shorter the exposure wavelength to be used is, the higher the resolution of the projection optical system is. The larger the numerical aperture of the projection optical system is, the higher the resolution of the projection optical system is. Therefore, the exposure wavelength, which is used for the exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used in some situations. When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA   (1)δ=±k2·λ/NA2   (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ is too narrowed, it is difficult to match the substrate surface with respect to the image plane of the projection optical system. It is feared that the margin is insufficient during the exposure operation. Accordingly, the liquid immersion method has been suggested, which is disclosed, for example, in International Publication No. 99/49504 as a method for substantially shortening the exposure wavelength and widening the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent to form a liquid immersion area so that the resolution is improved and the depth of focus is magnified about n times by utilizing the fact that the wavelength of the exposure beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases).
The optical sensor (light receiver) described above has the light-transmitting section which is arranged on the image plane side of the projection optical system, wherein the light is received through the light-transmitting section. Therefore, when the numerical aperture of the projection optical system is increased as a result of the adoption of the liquid immersion method or the like, and the incident angle of the exposure beam (angle formed by the outermost ray and the optical axis) is increased, then the expansion of the light outgone from the light-transmitting section is increased as well, and it is feared that the light cannot be received satisfactorily.